Current nonvolatile semiconductor memory devices, as typified by electrically rewritable EEPROM (electrically erasable and programmable ROM), include those having a laminate structure, called SONOS (silicon-oxide-nitride-oxide-silicon) type or MONOS (metal-oxide-nitride-oxide-silicon) type. In these types of nonvolatile semiconductor memory devices, holding of information is performed with a silicon nitride film (nitride), sandwiched between silicon dioxide films (oxide), as a charge storage layer. In particular, in such a nonvolatile semiconductor memory device, by applying a voltage between a semiconductor substrate (silicon) and a control gate electrode (silicon or metal), electrons are injected into a silicon nitride film as a charge storage layer to store data, or electrons stored in the silicon nitride film are removed to erase data. Rewriting of data is thus performed.
As a technique for forming a silicon nitride film as a charge storage layer of a nonvolatile semiconductor memory device, Japanese Patent Laid-Open Publication No. 5-145078 (patent document 1) describes a method for forming a silicon nitride film between a tunnel oxide film and a top oxide film by a reduced pressure CVD method using dichlorosilane (SiH2Cl2) and ammonia (NH3) as source gases, carried out at the flow rate ratio SiH2Cl2/NH3 of not more than 1/10.
With the recent higher integration of semiconductor devices, the device structures of nonvolatile semiconductor memory devices are becoming increasingly miniaturized. To miniaturize a nonvolatile semiconductor memory device, it is necessary to enhance the charge storage capacity of a silicon nitride film as a charge storage layer in the memory device, thereby enhancing the data storage capacity. The charge storage capacity of a silicon nitride film is related to the density of traps, which serve as a charge capture center, in the film. The use of a silicon nitride film having a large trap density as a charge storage layer is therefore considered effective as a means for enhancing the data storage capacity of a nonvolatile semiconductor memory device.
However, it is technically difficult with a conventional reduced CVD or thermal CVD film forming method to control the trap density of a silicon nitride film in the course of the formation of the film; and the formation of a silicon nitride film having a desired trap density has not been attained as yet. For example, it is not possible with the above-described silicon nitride film-forming method disclosed in the patent document 1 to directly control the trap density of a silicon nitride film. Therefore, in order to increase the trap density at the interface between the silicon nitride film and a top oxide film, an intermediate transition layer containing a large amount of Si is provided between the two films. Further, the method of the patent document 1 employs a technique to control the timings of supply of source gases in a complicated manner. In particular, according to the technique disclosed in the patent document 1, when terminating the formation of a silicon nitride film, the supply of ammonia is first stopped and, after the remaining ammonia is consumed, the supply of dichlorosilane is stopped. When later forming a top oxide film, only nitrous oxide is first supplied and, after the elapse of a predetermined period of time, silane is supplied to initiate deposition of the top oxide film.
However, when such a method of controlling the timings of supply of source gases as described in the patent document 1 is employed, a slight timing difference will cause a large change in the quality of the resulting film. Therefore, it appears totally impossible to form a silicon nitride film having a desired trap density with good reproducibility by such a film forming method.